When digital data are transmitted serially over radio, wire, fiber-optic, or other links, it is often necessary to transmit and receive frame synchronization information so as to identify groups of the received data bits which belong to a distinct function or represent bits related to each other in some particular way. One such example is the organizing of 8, 16, or 32 bit groups into individually addressable words for computer storage and retrieval. Another example groups together 16 adjacent bits which numerically represent the voltage level of an analog audio waveform as sampled at a particular instant. For these systems, additional bits are appended to the data which may then be detected by a receiver to achieve synchronization.
For example, data transmitted over an RS-232 interface comprises 8 data bits plus a start bit and one or two stop bits. Therefore, in this system, there are 8 data bits per frame as well as two or three synchronization bits per frame. The two or three synchronization bits are considered overhead, which slow down the effective information data rate of the RS-232 interface.
Such systems involving frame synchronization bits often complicate decoding, reduce power available for information bits, and introduce line spectra in the emitted signal.
The Institute of Gas Technology (IGT) has disclosed a system that introduces errors into the data stream as described in the report: Using the SRT 24-12-03 to Improve Data Security and Reliability in an Automatic Meter Reading Application, by L. Rennie (November, 1987). In this system, data transmission security is enhanced by inserting random errors into a coded channel before transmission. Upon reception, these errors are detected and corrected, but they serve to obscure the transmitted data for unauthorized listeners.
In the IGT system, a problem exists in that no means is provided to establish frame synchronization beyond that inherent in the block code.
Accordingly, it is desirable to have a system in which deliberate errors are introduced into the digital data stream at specific instances, thereby eliminating the need for frame synchronization bits and allowing for transmission of the digital data stream as a low probability of intercept signal. It is also desired to have a system in which noise-like data can be transmitted and received by taking advantage of the different frame sizes between a digital data stream frame size and an encoding frame size.